Active suspending

ABSTRACT

An apparatus has an active suspension including an electromagnetic actuator coupled to a plant in a vehicle. A force bias eliminator is coupled to the plant for causing the actuator to experience a zero-mean load, and a vibration isolation block generates a control signal based on the response of a nominal plant to measured disturbances of the actively suspended plant. A compensation system modifies the control signal in response to a difference between the response of the nominal plant to the control signal and a measured response of the actively-suspended plant to the control signal.

This application is a divisional of and claims the priority benefit ofearlier-filed application Ser. No. 10/978,105, filed on Oct. 29, 2004,entitled ACTIVE SUSPENDING; and application Ser. No. 11/418,345, filedon May 3, 2006, entitled ACTIVE SUSPENDING, the entire disclosure ofwhich is hereby incorporated by reference.

FIELD OF INVENTION

This description relates to active suspending.

BACKGROUND

A vehicle moving in a desired direction inevitably experiences motion inother directions as well. This undesired motion often arises fromdisturbances in the medium through which the vehicle travels. Forexample, whether one travels by land, sea, or air, one might encounterbumps, waves, air pockets, and the like.

At best, such random acceleration causes discomfort and annoyance tothose in the vehicle. For certain susceptible individuals, these randomaccelerations can trigger a bout of motion sickness. However, in somecases, a particularly violent acceleration will cause the operator tobriefly lose control of the vehicle.

Even when stationary, there is some residual vibration associated withthe vehicle's engine. In motion, even on smooth roads, this residualvibration can become oppressively tiresome.

SUMMARY

According to the present invention there is provided an apparatuscomprising: an active suspension including an electromagnetic actuatorcoupled to a plant in a vehicle; a force bias eliminator coupled to theplant for causing the actuator to experience a zero-mean load; avibration isolation block for generating a control signal based on theresponse of a nominal plant to measured disturbances of the activelysuspended plant, and a compensation system for modifying the controlsignal in response to a difference between the response of the nominalplant to the control signal and a measured response of theactively-suspended plant to the control signal.

The invention may also include the following features:

-   -   a sensor for providing information indicative of a state of the        plant.    -   a weight sensor for measuring a weight associated with the        plant.    -   the force bias eliminator comprises a pneumatic system.    -   the active suspension is configured to have a plurality of        operation modes, including an active mode and a passive mode.    -   the apparatus is configured to automatically switch between the        active mode and the passive mode in response to detection of an        event.    -   the apparatus is configured to switch between the active mode        and the passive mode in response to a user selection.    -   the plant comprises a seat, and wherein the electromagnetic        actuator is coupled to the seat for exerting a force on the        seat.    -   the compensation system is configured to modify the control        signal in response to differences in the weight of different        occupants of the seat.    -   the compensation system comprises a feedback loop carrying a        feedback signal indicative of plant acceleration; and the        compensation system is configured to maintain a constant        bandwidth of the feedback loop.    -   the compensation system is configured to modify the control        signal in response to a rapid change in the weight of the plant.    -   the compensation system is configured to modify the control        signal in response to a rapid change in the weight of the seat.    -   the control signal is modified so that the maximum actuator        force exerted is reduced while the force bias eliminator is        allowed to adapt to the new plant weight.    -   the control signal is modified so that the maximum actuator        force exerted is reduced while the force bias eliminator is        allowed to adapt to the new seat weight.    -   the compensation system is arranged to maintain a constant open        loop acceleration transfer function, independent of plant        differences.    -   the compensation system does not modify the control signal if        the control signal would cause the actuator to exert forces with        a magnitude on the order of frictional forces.    -   the compensation system does not modify the control signal if        the control signal would cause the acceleration of the plant to        be less than a predetermined threshold.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be apparent from thefollowing detailed description in which:

FIG. 1 and FIG. 4-7 show actively-suspended plants;

FIGS. 2 and 3 show parallel and series connections to actively-suspendedplants.

FIG. 8 shows a control system for controlling actively-suspended plantsof FIG. 1 and FIGS. 4-7.

FIG. 9 shows an embodiment of the control system of FIG. 8.

FIG. 10 shows a vibration isolation module;

FIGS. 11-12 show a control system with different types of plantestimators;

FIG. 13 is a flow-chart of an algorithm used by a force bias eliminator;

FIG. 14 shows an exemplary force bias eliminator;

FIG. 15 is a block diagram showing processing by the force biaseliminator;

FIG. 16 shows a control system in which the weight of the plant ismeasured;

FIG. 17 shows an algorithm for a fail-safe system;

FIG. 18 illustrates typical power demands;

FIG. 19 shows a power supply;

FIG. 20 shows a support for a seat that results in coupling betweenmotion along different axes; and

FIG. 21 shows a control system modified to accommodate coupling ofmotion along different axes.

DETAILED DESCRIPTION

An actively-suspended plant includes a seat, or other platform, coupledto one or more active suspension elements, each providing activesuspension along an axis. In many cases, it is useful, though by nomeans required, to have a passive suspension element cooperating with anactive suspension element along one or more axes. In such cases, theactive suspension element can be mounted either in series or in parallelwith the passive suspension element.

In the following description, numerous references are made to theposition and motion of a plant. It is understood, for facilitating thediscussion in the following in light of the disclosed embodiments, that“position” means position of the plant relative to a vehicle and that“motion” means motion of the plant relative to an inertial referenceframe. Accordingly, references to position signals refer to signals thatcarry information about the position of the plant relative to thevehicle. References to motion signals refer to signals that carryinformation about the motion, such as the acceleration, of the plantrelative to the inertial reference frame

The following description refers to embodiments in which the plant isintended to translate along any one or more of three coordinate axes ina Cartesian coordinate system. However, the control system does notrequire any particular coordinate system for its operation. For example,the plant can be configured to translate along any one or more of twoaxes. Additionally the plant can be configured to translate along one ormore axes that are non-orthogonal as well as orthogonal.

Neither is the control system described below restricted to the controlof plant translation. The control system can also be used to controlrotary motion, such as pitch, roll or yaw. Or, the control system can beused to control any combination of rotational and translational motion.

FIG. 1 shows an actively-suspended plant 10 having a vertical activesuspension element 12 for affecting, e.g., suppressing, motion of aplant 16 along a vertical axis, z, and a longitudinal active suspensionelement 14 for affecting motion of the plant 16 along a longitudinalaxis, y. Motion of the plant 16 along a transverse axis, x, is affectedby a passive suspension element 18, such as a spring or spring/damperbased system. In cases in which the plant 16 includes a seat, as in thecase shown in FIG. 1, the inclusion of an active suspension element 14for suppressing longitudinal motion, as opposed to inclusion of a purelypassive suspension, is particularly useful because it permits the seatto remain still when a force is exerted on the external environment.Such a feature is useful, for example, to prevent the seat from movingaft in response to pressing a foot pedal.

As used in this description, an active suspension is a suspension thatincludes an actuator as an integral part thereof. Such an actuator iscapable of generating forces whose magnitude and direction can becontrolled independently of the position and motion of the suspension.In some embodiments, the actuator is an electromagnetic actuator, orelectromagnetic motor, either linear or rotary, single-phase ormulti-phase.

The term “plant” is intended to include the system that receives acontrol signal and whose position and motion are to be controlled. Theplant can include a seat, a passenger, any fixtures associated with theseat, the seat's support structure, power electronics, and mathematicalmodels of active and/or passive suspension elements to the extent thatthose elements affect the dynamic properties of the system to becontrolled.

Actively-suspended plants can be used in a variety of applications. Forexample, an actively-suspended plant can be an engine mount, a platformon a boat, a seat, bed, or cab used in any moving vehicle such as a car,truck, boat or other watercraft, train, bus, recreational vehicle,ambulance, tractor, truck-trailer, farm machinery, constructionmachinery, weapons platform, airplane, helicopter or other aircraft, apersonal transportation device, such as a wheelchair, or a babycarriage. Other examples of actively-suspended plants include machinetool isolation tables, interferometer benches, photolithography tables,and the like.

The plant need not include a seat at all. It can, for example, be a bedfor sleeping, such as those found in truck cabs or in sleeping cars on atrain. Moreover, the plant need not carry a human being. For example,there exists cargo that is quite fragile (e.g. china and crystal) orquite explosive (e.g. dynamite), both of which are often transportedvery carefully. An actively-suspended plant would provide a suitable wayto transport such cargo.

Moreover, the plant may cover a significant area. For example, on aluxury cruise ship it may be useful to have a barber shop, or amotion-sickness recovery lounge, that stays stationary even as the shippitches and rolls. Such plants would enable one to enjoy the benefits ofa close shave even during a storm at sea.

Each suspension element, whether active or passive, suppresses motionalong at least one axis. In some embodiments, all axes can be providedwith active suspension elements, in which case no passive suspensionelements are needed. However, other embodiments include those in which:one axis, preferably the vertical axis, is provided with an activesuspension; and axes other than the vertical axes are provided withpassive suspension elements. Another option is to provide one axis,preferably a transverse axis, with a passive suspension and to providethe axes other than the transverse axes with active suspension. In otherembodiments, rotational motion, such as pitch, roll, and yaw arecontrolled. In such cases, an active suspension can be configured tourge the plant to pitch, roll, yaw, or any combination thereof.

Axes that lack active suspensions need not be provided with passivesuspensions at all. However, without passive suspensions in those axes,the passenger may experience discomfort. For this reason, it may bedesirable to provide one or more axes with both an active and a passivesuspension. In such cases, the active and passive suspensions can beplaced in series with the active suspension either below the passivesuspension, as shown in FIG. 2, or above the passive suspension (notshown). Alternatively, the active and passive suspensions can be placedin parallel, as shown in FIG. 3.

In some embodiments, the active suspension can be turned on and off,either indefinitely or from time-to-time, either for all axes at once,or on an axis-by-axis basis. In some cases, it may be useful to providea fail-safe system associated with the active suspension element todampen the motion of the plant along that axis should the activesuspension element associated with that axis fail.

Alternatively, if the active and passive suspensions are in series,power to the active suspension can be cut. In such cases, any movingparts associated with the active suspension can be clamped. Once themoving parts of the active suspension are clamped, only the passivesuspension will affect plant motion. In other cases, moving parts of theactive suspension remain free to move in response to the action ofpassive suspension elements.

The vertical active suspension element 12 includes: a verticalaccelerometer 20; a vertical position sensor 24; and a vertical actuator28 that, in the embodiments shown, is in parallel with the verticalposition sensor 24 and located preferably as closely as possible to thevertical accelerometer 20. Similarly, the longitudinal active suspensionelement 14 includes: a longitudinal accelerometer 22; a longitudinalposition sensor 26; and a longitudinal actuator 30 in parallel in thelongitudinal position sensor 26, and located preferably as closely aspossible to the longitudinal accelerometer 22.

Actuators 28, 30 that can be used in an active suspension element 12, 14include single or multi-phase electromagnetic actuators, such asthree-phase linear actuators, single phase linear actuators, rotaryactuators and variable reluctance actuators. One suitable actuator is anelectromagnetic linear actuator such as that disclosed in U.S. Pat. No.4,981,309, the contents of which are herein incorporated by reference.

Any position sensor 24, 26 with sufficient resolution and accuracy canbe used. Examples of suitable position sensors 24, 26 include thosehaving potentiometers, those that make use of the Hall effect, and thosethat have magnetostrictive sensors. Examples of position sensors havingpotentiometers include those from Novotechnik Inc, Ostfildern, Germany.Other position sensors 24, 26 that can be used with an active suspensionelement include those having an encoder with a limit switch fordetermining absolute position. Variants of this type of position sensorcan also be used to derive acceleration relative to a reference framewhen a sensor fixed to that reference frame is available. Examples ofsuitable accelerometers 20 include MEMs (micro-electro-mechanical) basedaccelerometers, such as those in the KXM60 series manufactured byKionix, Inc., of Ithaca, N.Y.

To assist in suppressing its vertical motion, the actively-suspendedplant 10 includes an element for removing bias force in the actuatorcommand force signal so the actuator experiences zero-mean load. In someembodiments, this element has the dynamic characteristic of a variablelow stiffness spring. The low stiffness spring characteristic ensuresthat the actuator is not “fighting” a spring as it tries to performactive isolation. This reduces power consumption. Such an element, whichwill be referred to as a “force bias eliminator system” can beimplemented as an air cylinder having an associated reservoir, as shownin FIG. 14. The force bias eliminator system provides a biasing force,thereby relieving the actuator from supplying that force. Such biasesmay result from factors such as the weight of the plant 16. Because ofthe biasing force provided by the force bias eliminator system, thevertical actuator 28 need only suppress excursions from a predeterminedequilibrium position. In a preferred embodiment, the air cylinder and anassociated reservoir are configured such that the actuator sustains zeromean load. As discussed below, the force bias eliminator system can alsoprovide passive suspension, either with or without additional damping.

Another embodiment of an actively-suspended plant 16, shown in FIG. 4,provides a single active suspension element 34 oriented to suppressmotion of a plant 16 along a vertical axis. The active suspensionelement 34 in this case is mounted in series with a multi-axis passivesuspension element 36. As shown in FIG. 4, the multi-axis passivesuspension element 36 is mounted between the active suspension element34 and the plant 16. However, the passive suspension element 36 can alsobe mounted between the plant 16 and the vehicle floor 37, as shown inFIG. 5.

As shown in FIG. 6, a plant 16 can include various features orstructures other than a seat 40 and its occupant. These additionalfeatures or structures are of the type that benefit greatly by beingheld stationary relative to the plant 16. Exemplary structures include acup-holder 42, which often hold drinks susceptible to spillage inresponse to random accelerations of the vehicle, a writing surface, adata entry/retrieval device, an ashtray or other receptacle 44, adisplay, such as a navigation display, and controls 47, particularlycontrols that do not require a direct mechanical linkage to the vehicle.Exemplary controls include electronic controls for operation of heavyequipment, and controls, such as pedals or levers, for braking andacceleration. Although as shown the features or structures are attachedto the plant 16, it should be noted that features or structures may beremotely located (not shown) from the plant 16 but “slaved” to themotion of the plant 16.

The actively-suspended plants 16 shown in FIG. 1 and FIGS. 4-7 include abase 46 that is configured to bolt into standard bolt patterns found invarious makes and models of motor vehicles. However, theactively-suspended plants 16 can be supported by any of a variety ofsupport structures, including a scissors mechanism 51 as shown in FIG.7, a modified scissor mechanism, a four-bar linkage, and a modifiedfour-bar linkage to be used when the actuator is one for which the ratioof actuator stroke to seat travel is less than unity. Moreover, theactuators themselves can be designed as part of the support structure.

The active suspension elements 12, 14 are in data communication with acontrol system 48, shown in more detail in FIG. 8. The control system 48receives data signals, such as plant acceleration a_(n) and the positionp_(r) of the plant 16 relative to the vehicle, from the sensors 24, 26.In return, the control system 48 provides, via a controller 49, controlsignals u_(r) for causing the respective actuators 28, 30 (see FIG. 1)to exert forces that tend to: restore the plant 16 to an equilibriumposition; and minimize the acceleration experienced by the plant 16. Thedata signals can represent position and acceleration of the plant 16, aswell as data indicative of properties of the plant 16. A force biaseliminator module 60 in communication with the plant 16 removes the biasfrom the actuator force control signal u_(r) so as to maintain zero meanload to the actuator.

In practice, the relevant characteristics of the real plant 16 may notbe known precisely. In general, therefore, the design of any controller49 and the resulting control signal (u_(r)) output by that controller49, would be based on assumptions about those characteristics. As aresult, the control signal used to control the real plant 16 may notachieve the expected result. Therefore, the control system 48 estimateserrors in the assumptions concerning the real plant 16 and compensatesfor those errors. In some embodiments, such as those described below,this estimation and compensation makes use of a reference model.

FIG. 9 shows an exemplary control system 48 in which a reference modelincludes a mathematical reference model 50 of a nominal plant in theform of that nominal plant's response, Pn(s), to complex frequencyinputs, s. For brevity of expression, this mathematical model 50 of thenominal plant will be referred to simply as the “nominal plant 50.” Theresponse, Pn(s), of the nominal plant is used by the vibration isolationmodule 52, together with data indicative of a real plant's position andmotion, to calculate a nominal control signal u_(n).

The nominal plant 50 is thus a reference model for the real plant 16.Such a model can be defined to include one or more parameters,including: desired performance characteristics, frequency response,poles/zeros, or any combination thereof. For example, in the case of areal plant 16 that includes a vehicle seat, a parameter representing anominal driver weight can be defined as the average weight across alarge number of representative drivers.

A variety of methods are available for implementing the vibrationisolation module 52. For example, FIG. 10 shows a vibration isolationmodule 52 that includes a position and an acceleration feedback signal.In some implementations, a position controller having a positionfeedback loop takes a relative position signal p_(r) as input and urgesthe real plant 16 to maintain a predetermined equilibrium position r. Insome cases, the equilibrium position may correspond to the midpoint ofan actuator stroke. In other implementations, an acceleration controllerhaving an acceleration feedback loop takes an acceleration signal a_(r)as input and controls the acceleration experienced by the real plant 16.

To facilitate discussion, the following description is based on atwo-loop control structure. However, in general, the controller usesboth position and acceleration of the real plant 16 as inputs andprovides a control signal as an output. The implementation need not be atwo-loop controller structure. For example, the controller can have asingle loop. Other embodiments include controllers having 2n inputs andn outputs, where n is the number of actively controlled axes.

The bandwidth of the position loop can be designed on the basis of apassenger's perception of discomfort. This varies with the particularaxis along which movement of the real plant 16 is to be controlled. Forexample, most passengers tolerate greater vibration in the verticaldirection than in the fore-aft direction. In addition, the spectrum ofvibration in the fore-aft direction generally has larger high-frequencycomponents than does the spectrum of vibration in the verticaldirection. Accordingly, in some embodiments, a position loop used tosuppress vibration in the vertical direction has a smaller bandwidththan does a position loop used to suppress vibration in the horizontaldirection. When more than one axis is actively controlled, the positionloop for each active axis can have a bandwidth tailored to accommodatethe vibration characteristic along that axis.

In the implementation shown in FIG. 10, the measured position signal,p_(r), which represents the relative displacement between the real plant16 and the vehicle's frame, is subtracted from a desired equilibriumposition, r, of the plant 16. The resulting difference is provided as aninput to the vibration isolation module 52. The measured accelerationsignal, a_(r), is subtracted from a desired acceleration, which in theillustrated embodiment is a zero acceleration. The resulting differenceis provided as an input to the vibration isolation module 52. The outputof the vibration isolation module 52 is the nominal control signalu_(n). Suitable vibration isolation modules 52 include those describedin U.S. Pat. Nos. 3,701,499 and 6,460,803, the contents of which areherein incorporated by reference.

In some cases, it may be desirable to change the desired equilibriumposition r of the real plant 16 while actively controlling the plant 16.For example, when the real plant 16 includes a seat, it may be desirableto adjust the seat height to accommodate different occupants. This canbe done by changing the desired equilibrium position r. For the controlsystem 48 shown in FIG. 9, the change in the described equilibriumposition, r, will cause a bias force component in the control signal,u_(r). The need for this bias force component is removed by the forcebias eliminator module 60.

As noted above, the vibration isolation module 52 generates a nominalcontrol signal that would be used to control motion of a nominal plant50 were that nominal plant 50 to experience certain disturbancesrepresented by the measured position and acceleration signals. In orderto make the vibration isolation module 52 generate a nominal controlsignal, signals a_(r), p_(r) indicative of these disturbances areprovided as inputs to the vibration isolation module 52. However, ingeneral, the nominal plant 50 has dynamic characteristics that differfrom those of the real plant 16. Therefore, the output of the vibrationisolation module 52 will, in general, not be optimized for controllingthe movement of the real plant 16 subject to those same disturbances.

In most cases, however, the nominal plant 50 and the real plant 16 havesimilar enough dynamic characteristics so that a control signal forcontrolling the real plant 16, referred to herein as the “real controlsignal” is similar to the nominal control signal.

It is important to note that there is no actual nominal plant 50 that isundergoing any actual physical movement. What there is a model of anominal plant. This model is selected to respond in a manner similar tohow a real plant 16 might respond.

In effect, the control system 48 uses the nominal plant 50 to simulatethe response of the real plant 16 to a control signal. The controlsystem 48 has, at its disposal, the hypothetical response of the nominalplant 50 to a control signal and the actual measured response of a realplant 16 to that control signal. On the basis of a difference betweenthe hypothetical response and the actual measured response, the controlsystem 48 adjusts the control signal.

To compensate for the difference between the real plant 16 and thenominal plant 50, the control system 48 includes a plant estimator 62that estimates this difference based at least in part on signalsindicative of the motion experienced by the real plant 16. The plantestimator 62 then provides an error signal e(s) representative of thatdifference to a plant compensator 64. The plant compensator 64 thencompensates for the difference by modifying the nominal control signalu_(n) before applying it to the real plant 16. The combination of theplant estimator 62 and plant compensator 64 is referred to as a“compensation system 65.” Although the plant estimator 62 andcompensator 64 are shown as being separate from each other, this is doneonly to illustrate their separate functions. In practice, the functionsof a plant estimator 62 and compensator 64 can be carried out bycircuitry embodied in a single hardware element, or in software.

In essence, the plant compensator 64 uses the error signal to perturbthe nominal control signal, u_(n). The result of that perturbation isthe real control signal, u_(r), which is applied to the real plant 16.As shown in FIG. 11, the plant compensator 64 includes a multiplier.However, the plant compensator 64 can also include a filter. Note thatas used herein, “real” indicates that the control signal is to beapplied to the real plant 16. It does not have its usual mathematicalmeaning of a signal having no imaginary component.

In FIG. 9, the plant estimator 62 is shown as accepting a variety ofinputs, among which are: real disturbance signals indicative of thedisturbance experienced by the real plant 16, e.g. position andacceleration signals a_(r), p_(r) and nominal disturbance signals 51indicative of corresponding disturbances that would be experienced by anominal plant 50 being controlled by the nominal control signal u_(n).Alternatively, other information obtained from outside the controlsystem 48 can be used to estimate the difference, as discussed below.These inputs represent potential sources of information that the plantestimator 62 can use to generate an error signal. Embodiments of theplant estimator 62 need not actually receive or make use of all theinformation sources shown in FIG. 9, but may instead receive or make useof a subset of those sources.

The details of designing the compensation system 65 depend on thecontrol objective to be achieved. In one embodiment, shown in FIG. 11,the control objective for the compensation system 65 is to maintain aconstant bandwidth of the open acceleration loop transfer function, asdefined by the frequencies at which the magnitude of the openacceleration loop transfer function crosses the 0 dB line, independentof any plant differences such as those caused by the passenger weight.The compensation system 65 does so by adaptively adjusting the gain ofthe acceleration loop transfer function in response to a differencebetween a real acceleration experienced by a real plant 16 and a nominalacceleration that would be experienced by a nominal plant 50 under thesame circumstances.

A suitable compensation system 65 in such a case is a model referenceadaptive controller. In this case, the plant estimator 62 generates anerror signal, e(s), on the basis of a real acceleration signal a_(r)from the real plant 16 and a nominal acceleration signal u_(n)representing the acceleration that would have been experienced by thenominal plant 50. The plant compensator 64 in this case is a multiplierthat multiplies the nominal control signal, u_(n), with the errorsignal, e, to obtain the real control signal, u_(r).

As shown in FIG. 12, a compensation system 65 implemented using a modelreference adaptive controller includes a first filter 66 that filtersthe nominal acceleration, a_(n), that a nominal plant 50 would haveexperienced under the same circumstances, and a second filter 68 thatfilters the actual acceleration, a_(r), experienced by the real plant16. The nominal acceleration a_(n) is obtained as an output of thenominal plant 50 when using nominal control signal, u_(n) as input.

The output of the first filter 66 is the filtered nominal acceleration,a_(fn), and the output of the second filter 68 is the filtered actualacceleration a_(fr). The first and second filters 66, 68 are centered onthe desired cross over frequency (i.e., the 0 dB frequency).

The filtered nominal acceleration is subtracted from the filtered actualacceleration at a subtractor 70 to generate an error signal. This errorsignal can be further processed in a variety of ways to minimize somequantity indicative of the error e.

In the embodiment shown in FIG. 12, the quantity indicative of the errorto be minimized is the least mean square (LMS) of the error e. This isachieved by multiplying the error signal with the filtered nominalacceleration, a_(fn), at a multiplier 72. The result of this operation,which is the derivative of the compensation signal is then provided toan integrator 74. The output of the integrator 74 is then multipliedwith the nominal control signal to generate the real control signal.

An optional feature of the compensation system 65 is that of causing theintegrator 74 to provide unity output under certain specialcircumstances. Under these special circumstances, when the output of theintegrator 74 is multiplied with the nominal control signal, the nominalcontrol signal would remain unchanged. Hence, the real control signalwould be the same as the nominal control signal. Exemplary specialcircumstances would include the detection of a nominal control signalthat would cause the actuator to exert very small forces, on the orderof frictional forces. Other special circumstances include detecting anacceleration that is below a threshold, or any combination thereof.

FIG. 12 shows a specific embodiment of a compensation system 65 thatcompensates for just one of the many factors, in this case a change inthe dynamic properties of the real plant 16. Such a change might result,for example, from differences in passenger weight that might result in adifference between the dynamic properties of a real plant 16 and anominal plant 50. However, the compensation system 65 can be designatedto compensate for other such factors. One such factor includes drift inpower electronics parameters.

The control system 48 is an analog system that makes use of continuoustime signals. However, the control system 48 can also be implemented indiscrete time, in which case the integrator 74 becomes a summation blockand the low-pass filters 66, 68 become suitably defined digital filters.

As noted above, the vertical active suspension element 12, whichsuppresses motion in the vertical direction, includes a verticalactuator 28 that exerts the forces necessary to maintain the plant 16 atan equilibrium vertical position. However, in the vertical direction,the plant 16 is constantly subjected to the force of gravity. As aresult, the actuator 28 consumes considerable energy simply supportingthe weight of the plant 16.

In one implementation, a force bias eliminator system is provided toexert a bias force in the vertical direction that is sufficient tooffset the force bias component in the real control signal, u_(r),thereby maintaining zero mean load for the vertical actuator 28. Withthe force bias eliminator system thus available, the vertical actuator28 is spared having to exert a force simply to hold the plant 16 in itsequilibrium position. Instead, the vertical actuator 28 need only exertforces to compensate for brief excursions from the equilibrium position.

A force bias eliminator system as described above is not strictlynecessary. In principle, one could simply cause the vertical actuator 28to exert a suitable bias force. Such a configuration may be practicalif, for example, a room-temperature superconductor were available tocarry the current required to generate such forces. However, for knownelectromagnetic actuators, the currents required to support a plant 16would be uncomfortably large, and would generate considerable wasteheat.

The force bias eliminator system can be a relatively simple one, such asan adjustable spring or a device that has the mechanical properties of alow stiffness adjustable spring.

A suitable force bias eliminator system preferably operates whether ornot the vehicle is turned on. This will enable the occupants of thevehicle to remain comfortably seated with all power shut down. Such afeature is also important for safety. It would be most disconcerting if,in an automobile traveling at highway speeds, loss of power wereimmediately followed by a sudden drop in the position of all seats.

To provide control over this force bias eliminator system, the controlsystem 48 also includes a force bias eliminator module 60 (see FIG. 11)whose function is to cause the force bias eliminator system to provide asuitable bias force under a variety of changing circumstances. As shownin FIG. 9, the force bias eliminator module 60 receives acceleration andposition data from the real plant 16 as well as the real control signal,u_(r), from the compensation system 65. On the basis of this data, theforce bias eliminator module 60 provides a bias control signal to theforce bias eliminator system, as discussed below in connection with FIG.13.

As described below, the active suspension system is configured tooperate in a plurality of modes: a safe (passive/failsafe) mode, anactive (force bias elimination) mode, and a bump stop mode. As shown inFIG. 13, the system, via the force bias eliminator module 60 or aseparate fail-safe system (see details below), first detects theoccurrence of a trigger event. A trigger event can occur in response toany change in a characteristic of plant 16 that may indicate an abnormalstate. Exemplary trigger events include failure of an active suspensionelement, a severing of a power cable, or a sensor failure (step 76).Upon detection of a trigger event, the force bias eliminator module 60causes the force bias eliminator system 86 to operate in a mode referredto as “passive mode,” “safe mode,” or “fail-safe mode” (step 78). Inthis mode, the position of the plant 16 is adjusted via the force biaseliminator module 60, as discussed below in connection with FIGS. 14 and15. In some embodiments, switching to “passive mode” operation can alsobe implemented as a user-selectable feature. Whether or not the activesuspension elements are operating can readily be determined by, forexample, detecting power being supplied to them. If the systemdetermines that the active suspension elements are currently operating,it then uses the acceleration and position signals from the real plant16 to determine whether the vertical actuator 28 is likely to reach theend of its travel, i.e. whether the vertical actuator 28 is likely tostrike one of its two bump stops (step 80). If so, the force biaseliminator module 60 causes the force bias eliminator system to operatein “bump-stop” mode (step 82). Otherwise, the force bias eliminatormodule 60 causes the force bias eliminator system to operate in normalmode or “active mode” (step 84), as discussed below in connection withFIGS. 14 and 15, in which the position of the plant 16 is adjusted bycontrolling one or more actuators.

An exemplary force bias eliminator system 86 is a pneumatic force biaseliminator (shown in FIG. 14) that includes a cylinder 88 and a matchingpiston 90 on which the plant 16 is supported. The cylinder volume belowthe piston head, i.e. the “lower cylinder chamber,” is connected eitherto a compressed air source (not shown), by way of a supply valve 92, orto ambient air, by way of a bleed valve 94. Alternatively, the lowercylinder chamber can be connected to either a compressed air source (notshown) or to ambient air by operating a three-way manual adjustmentvalve 96. The compressed air source can be a readily available on-boardair source, such as a reservoir of compressed air maintained at highpressure by a pump. Hollow portions of the seat structure can also beused as air reservoirs, thereby incorporating, or integrating, the airreservoir in the seat structure itself. Alternatively, the force biaseliminator system can be a hydraulic system.

The cylinder 88 can include a piston 90 that moves in response to airpressure and to the plant weight. Or, the cylinder 88 can simply expandand contract in response to pressure and weight, in much the same waythat a rubber tire will expand and contract. An expansion chamber 98,which is in fluid communication with the air cylinder 88, can be anexternal air-reservoir. Alternatively, the expansion chamber 98 can bebuilt into the seat structure itself, thereby conserving space withinthe vehicle interior.

In normal mode or “active mode,” the force bias eliminator module 60determines, based, for example, on a control signal u_(r) as shown inFIG. 11, whether the pressure needs to be increased or decreased. If thepressure needs to be increased, the force bias eliminator module 60causes the supply valve 92 to open and the bleed valve 94 to close,thereby flooding the lower cylinder chamber with compressed air.Conversely, if the pressure needs to be decreased, the controller causesthe supply valve 92 to close and the bleed valve 94 to open. This bleedshigh-pressure air from the lower cylinder chamber.

FIG. 15 shows the process of detecting and removing the bias componentof the real control signal by using the real control signal u_(r) as aninput to the force bias eliminator module 60 operating in active mode.The real control signal u_(r) is first passed through a low-pass filter100 to remove high frequency variations that are more likely to be theresult of attempts to cancel random accelerations. The low-pass filter100 thus isolates the low frequency variations that are more likely tobe the result of actual weight changes in the plant 16. A suitable lowpass filter 100 is one having a corner frequency on the order of 0.5 Hz.

The force bias eliminator module 60 then uses the sign, or phase angle,of the low frequency components of the real control signal to determinewhether to exert a bias force to offset the bias signal components inu_(r). For the implementation of FIG. 15, the force bias eliminatormodule 60, which takes the real control signal u_(r) as an input,determines whether pressure against the piston 90 needs to be increasedor decreased. The force bias eliminator module 60 then sends appropriatevalve-actuation signals V to a supply valve relay (not shown) thatcontrols the supply valve 92 and to a bleed valve relay (not shown) thatcontrols the bleed valve 94. If the pressure needs to be increased, theforce bias eliminator module 60 sends a signal to the supply valve 92relay to open the supply valve and a signal to the bleed valve relay toclose the bleed valve 94. Conversely, if pressure needs to be decreased,the force bias eliminator module 60 sends a valve-actuation signal V tothe bleed valve 94 relay to open the bleed valve and a signal to thesupply valve relay to close the supply valve 92. In some embodiments,relays with “backlash” (hysteresis) prevent chatter of the on-off valvesaround the relay's setpoint.

The force bias eliminator system 86 also includes upper and lower bumpstop valves 102, 104 that are used, in “bump-stop” mode, to resistmovement in those circumstances in which the vertical actuator 28 isunlikely to prevent the plant 16 from abruptly reaching the end of itstravel.

The upper bump stop valve 102 provides a path between the cylindervolume above the piston head (the “upper cylinder chamber”) and theambient air. In normal operation, this upper bump stop valve 102 is leftopen so that air can move freely in or out of the upper cylinderchamber. However, if the force bias eliminator module 60 detects thatthe vertical actuator 28 is unlikely to be able to stop the plant 16from reaching the top of its travel, it closes the upper bump stop valve102. This prevents air from escaping from the upper cylinder chamber asthe piston 90 moves upward. As a result, the air is compressed as thepiston 90 travels upward, thereby exerting a force that tends to resistfurther upward movement of the piston 90 (and hence the plant 16).

The lower bump stop valve 104 provides a path between the lower cylinderchamber and either the ambient air or the compressed air supply,depending on which of the bleed valve 94 and the supply valve 92 is openand which is shut. In normal operation, the lower bump stop valve 104 isleft open. This permits the force bias eliminator module 60 to freelycontrol the plant height by selectively opening and closing the bleedvalve 94 and the supply valve 92. However, if the force bias eliminatormodule 60 detects that the vertical actuator 28 is unlikely to stop theplant 16 from reaching the bottom of its travel, it closes the lowerbump stop valve 104. This prevents air from escaping from the lowercylinder chamber as the piston 90 moves downward, thereby exerting aforce that tends to resist further downward movement of the piston 90(and hence the plant 16).

When the force bias eliminator module 60 determines that the activesuspension element has been disabled, it sends a valve actuation signalV to seal off the upper and lower chambers by closing the upper andlower bump stop valves 102, 104 simultaneously. This causes the forcebias eliminator system 86 to operate in “safe mode,” which is a mode inwhich the force bias eliminator system 86 functions as a spring. Whenoperating in safe mode, the only way for air to enter and leave thecylinder 88 is through the three-way manual adjustment valve 96. Thethree-way manual adjustment valve 96 has: a closed position, in which noair can enter or leave the lower chamber; a bleed position, in which thelower chamber is connected to ambient air; and a fill position, in whichthe lower chamber is connected to a compressed-air source (not shown).

In safe mode, the level of the plant 16 is controlled by operating theadjustment valve 96. To raise the plant 16, the adjustment valve 96 ismade to connect the compressed air supply to the lower cylinder chamber.To lower the plant 16, the adjustment valve 96 is made to connect theambient air with the lower cylinder chamber. When the plant 16 is at thedesired position, the adjustment valve 96 is made to seal the lowercylinder chamber.

The following table summarizes the configuration of the various valvesshown in FIG. 14 during various operation modes:

FORCE BIAS UPPER LOWER SAFE ELIMINATION BUMP BUMP MODE MODE STOP STOPLower bump Close Open Open Close stop valve 104 Bleed valve 94 Open Opento lower Close Close or close Close to raise Supply valve 92 Close Opento raise Close Close Close to lower Upper bump Close Open Close Openstop valve 102 3-way valve 96 3 way Close Close Open

As noted above, during force bias elimination mode, a compressed airsource can be in communication with the lower chamber of the cylinder88. Should the vertical actuator 28 need to lower the plant 16momentarily, it may find it difficult to do so because the compressedair source will resist downward motion of the plant 16. To enable thevertical actuator 28 to more easily lower the plant 16, the force biaseliminator system includes an expansion chamber 98 disposed between thesupply valve 86 and the lower bump stop valve 104. The expansion chamber98 functions as a weak spring so that should the vertical actuator 28have to lower the plant 16, it will encounter minimal resistance fromthe compressed air source.

When a plant parameter changes suddenly, for example when an occupantsits down or stands up, or when cargo is removed or added, the maximumforce that the actuator would need to exert can be reduced, therebyreducing power usage. This is achieved by having the control system waitfor the force bias eliminator system to adapt to the new load. After theforce bias eliminator system has adapted, the actuator can be made toexert a reduced force needed for normal operation. For example, when aseated occupant stands up, the force bias eliminator system, whenimplemented as an air spring, will dump pressure quickly. Once itcompletes dumping pressure, the actuator is made to exert whateverforces are needed to actively suspend the now unoccupied seat. As aresult, the seat height remains approximately constant when the occupantarises. By contrast, a seat supported by a conventional spring willspring back up to an unloaded position when the occupant.

The force bias eliminator system 86 disclosed in connection with FIG. 14is a pneumatic system that implements pneumatic logic to carry out itsoperation. However, other types of force bias eliminator systems, suchas hydraulic systems can be used to apply a bias force to the plant 16.

The pneumatic force bias eliminator 86 described above can be used toimplement embodiments of the plant estimator 62 that measure the mass ofthe plant 16 directly. In such embodiments, shown in FIG. 16, a biaspressure is measured by a pressure sensor 108 and excursions from thebias pressure, after suitable filtering, indicate the weight of theplant 16. The bias pressure is the pressure that results from supportinga nominal plant 50. This pressure can be measured at the factory orfollowing seat installation and can be programmed into the plantestimator 62. The pressure sensor 108 measures pressure at any point inthe pneumatic system at which there exists a measurable pressure thatdepends on the weight of the plant 16. The pressure measured by thepressure sensor 108 is then passed through a low-pass filter 110 toeliminate any jitter. The result is then provided to the plant estimator62, which determines an error signal to be used for perturbing thenominal control signal.

The above description has focused on reference-model-based controldesign as shown in FIG. 9. However, other embodiments include thegeneral control system shown in FIG. 8, in which the controller can bedesigned via suitable linear or nonlinear control methods, with orwithout a reference model. For example, the vibration isolation modulein FIG. 9 can replace the controller module 52 in FIG. 8 without theneed to provide a nominal plant 50, a compensation system 65 or a forcebias eliminator module 60.

A failure in an active suspension element, particularly a verticalactive suspension element 12, may result in a sudden and possiblyalarming change in the plant's position and motion. To avoid this, thecontrol system 48 can be provided with a fail-safe system controlled bya failure-detector 112, as shown in FIG. 9. The fail-safe system has aselectively-activated damper coupled to the actuator. The damper may bea separate element. Alternatively, the damper may be implemented bychanging characteristics of an active suspension element. Under normalcircumstances, the damper is deactivated and therefore generates nodamping force. However, if the failure detector 112 detects theexistence of a particular condition, it activates the damper, therebycausing a damping force that resists motion of the plant 16. This causesthe plant 16 to settle gracefully to the lower bump stop. Alternatively,the fail-safe system can include a spring or a structure that functionsas a spring. One such structure is the force bias eliminator system asdescribed earlier. In such a case, the plant 16 will settle to anequilibrium position above the lower bump stop.

As shown in FIG. 9, the failure detector 112 is provided withinformation indicative of the state of the real plant 16. Thisinformation can include, for example, the position and accelerationsignals. On the basis of this information, the failure detector 112determines whether it is necessary to dampen the motion of the plant 16.A suitable damper for use in connection with an actuator 12 is describedin U.S. Pat. No. 4,981,309, the contents of which are hereinincorporated by reference.

Failure of the active suspension system is not the only reason toactivate the fail-safe system. Any change in a characteristic of theplant 16 that may indicate an abnormal state may be a reason to activatethe fail-safe system. For example, a sensor signal larger than apredetermined threshold, or a failure in any of the sensors that collectinformation indicative of the state of the plant, would be reasons toactivate the fail safe system. Sensor or system failure can be detectedby noting the absence or fluctuating presence of such signals, or bysensor signals that provide information inconsistent with physicalconstraints on the plant. For example, if a sensor were to indicate thatan automobile was now moving at supersonic speeds, the reliability ofthat sensor might reasonably be called into question, in which case thefail-safe system would be activated. Alternatively, detecting that asensor has reached the end of its useable range can also activate thefail-safe system. In some embodiments, the particular trigger event thatcauses transition from active mode to fail-safe mode, or “passive mode,”can also be implemented as a user-selectable feature.

In the case of an electromagnetic actuator, a stator having a coil ofwire surrounds an armature on which the real plant 16 is mounted. Thestator and armature together form an “electromagnetic actuator,” withthe position of the armature being controllable by the current in thecoil of the stator. In normal operation, the current through the coilgenerates a magnetic field that controls the position of the armature.Upon detection of failure, the leads of the coil are shorted, or clampedtogether. Under these circumstances, Lenz's law will operate to induce acurrent in the coil that generates a magnetic field tending to resistmovement of the armature. As a result, the electromagnetic actuatorfunctions as a damper.

FIG. 17 is an exemplary algorithm used by failure detector 112 indetermining whether to activate the damper following detection of asensor failure. The failure detector 112 monitors the accelerationsignal (step 114). If the acceleration signal is within pre-selectedlimits (step 116), the failure detector 112 assumes that the activesuspension element is operating correctly. Under these conditions, thedamper remains deactivated. The failure detector 112 then waits (step118) and inspects the acceleration again (step 114). However, if thefailure detector 112 detects that the acceleration signal is in excessof a threshold magnitude for longer than a pre-determined duration (step116), the failure detector 112 assumes that the active suspension hasfailed, and that remedial action is in order. Under these circumstances,the failure detector 112 activates the damper (step 120). The failuredetector 112 then stops further execution (step 122) until a resetoccurs.

The algorithm described in connection with FIG. 17 relies only on thedetection of a failure in the acceleration sensor. However, otheralgorithms can use the position signal, or a combination of the positionand acceleration signal, or any information indicative of the state ofthe real plant 16. Other algorithms for controlling the fail-safe systemuse information indicative of the condition of the various elements thatmake up the active suspension and associated components: such as thepower supply, the power amplifier, the controller itself, etc. Exemplaryinformation includes electrical bias signals to sensors, andelectro-motive forces generated by the actuator. Furthermore, redundantsensors can be used to improve system reliability.

The real plant control signal is ultimately used to modulate an outputcurrent of an amplifier 106, as shown in FIG. 18. This output current isultimately provided to an electromagnetic actuator 28. Fluctuations inthe control signal result in fluctuations in this output current.

The current to be modulated at the amplifier 106 (shown in FIG. 19) isprovided by a power supply 107. In an active suspension system of thetype described above, under normal operation on a smooth road, theamplifier 106 draws relatively small amounts of power. However, tocompensate for conditions such as large bumps on the road surface, theamplifier 106 requires short bursts of high power. FIG. 18 illustratesthe required power as a function of time for a typical urban streethaving occasional pot holes and other irregularities.

As shown in FIG. 18, in normal operation, the amplifier 106 draws anaverage power. However, occasionally, the amplifier 106 requiressignificantly more power, for short periods. To provide short bursts ofhigh power, it is useful to provide the amplifier 106 with a powersupply 107 having an energy storage element capable of providing shortbursts of high power without drawing directly on a power source, such asa battery.

A suitable power supply 107, shown in FIG. 19, includes a DC/DCconverter 109 having an input connected to a battery 113 and an outputconnected to a capacitance 110. The DC/DC converter 109 functions totransform the battery voltage, which is nominally 12 volts, into ahigher output voltage. An additional effect of the converter 109 is tolimit the amount of current drawn from the battery 113 by saturatingwhen the amplifier's demand for current exceeds that which the converter109 can provide. Under these circumstances, the converter 109 appears tothe amplifier 106 as a constant current source providing a saturationcurrent.

In general, in response to a change in parameters (such as outputcurrent, input power) of a converter circuit delivering power,additional power can be supplied by a passive power source, such as acapacitive element, to meet the peak power demand. A suitable powersupply circuit is disclosed in U.S. patent application Ser. No.10/872,040, filed on Jun. 18, 2004, the contents of which areincorporated by reference.

In normal operation, the converter 109 satisfies the amplifier'sappetite for current by itself. When the amplifier 106 requires morecurrent than the converter 109 can provide, it draws the deficit from acapacitance 110 that is connected in parallel with the output of theconverter 109. Since large currents are required only for relativelybrief periods, the charge stored in the capacitance 110 is sufficient tomeet the requirements of the amplifier 106. Since, as shown in FIG. 18,the demands for large current are separated by relatively long periods,the converter 109 can recharge the capacitance 110.

The particular numerical values associated with the components shown inFIG. 19 will depend on the details associated with each application.Such details include the mean current draw, the maximum current draw,the mean time between demands for large current draws, the operatingvoltage range of the amplifier 106, the converter's saturation currentand the parasitic resistance associated with the whatever arrangement ofcapacitors or other energy storage elements that are used to form thecapacitance 110.

The capacitance 110 need not be provided by a single capacitor. In somecases, it may be more economical to assemble capacitors into a circuithaving an appropriate equivalent capacitance 110 and the ability tooperate across the required voltage drop. For example, the circuit mayinclude many capacitors in series, with each capacitor having a ratedvoltage that is smaller than the power supply voltage applied toamplifier 106. In one embodiment, 62 capacitors, each having acapacitance of 17.3 farads and being rated to operate across a 2.5voltage drop, are placed in series to form the required capacitance.

Should a malfunction or instability occur, the force exerted by theactuator can have a tendency to grow very large and to stay large forlong periods of time. The power needed to maintain such a large forcefor long periods drains the capacitance 110 and causes the voltage tothe amplifier 106 to “droop.” This causes the amplifier 106 to disableitself. As a result, during instabilities, the power supply 107 imposesa power limitation that is low enough so that excess heat can bedissipated quickly, thereby avoiding thermal damage to the amplifier106. In this way, the illustrated power supply 107 limits the powerconsumption of the actuator should a malfunction or instability occur.In one embodiment, the capacitors are chosen such that the stored energycan be dissipated to disable the amplifier within 55 milliseconds shoulda malfunction or instability occur.

In those embodiments in which the plant includes a seat, a difficultycan arise because of the presence of a seat cushion. As described above,an accelerometer and a position sensor are mounted on the seat.Therefore, the accelerometer and the position sensor sense the motionexperienced by the seat. The assumption in this case is that a passengersitting on that seat will experience the identical motion as the seat.However, in most cases, the passenger does not sit directly on a seat.Instead, the passenger sits on a seat cushion. The cushion amounts to anadditional passive suspension element with its own transfer function.

In practice, the seat cushion introduces a transfer function having twozeros. These zeros reduce the gain of the acceleration loop and therebyreduce the control system's ability to suppress vibration at thefrequencies corresponding to those zeros.

To address this difficulty, the frequencies associated with the twozeros are made to exceed the bandwidth of the acceleration loopcontroller 58. This can be achieved by increasing the effectivestiffness of the springs within the cushion, for example by placing thecushion on a rigid backing.

The inputs and outputs of the various modules of the control system 48have thus far been viewed as scalars. This is because for many seatmounts, the force required to suppress vibration along one axis islargely independent of the force required to suppress vibration alonganother axis. In such cases, the plant 16 can be characterized by amatrix transfer function that is essentially diagonal, with very smalloff-diagonal components. The acceleration loop transfer function and theposition loop transfer function can also be characterized as essentiallydiagonal matrices. Under these circumstances, it is appropriate toconsider the suppression of vibration along one axis independently ofthe suppression of vibration along another axis.

However, for certain types of seat mounts, a force applied to suppressvibration along one axis may affect a simultaneous attempt to suppressmotion along a different axis. When this is the case, the plant transferfunction can no longer be viewed as a diagonal matrix.

For example, FIG. 20 shows a plant 16 that includes a seat supported bya four bar linkage. It is apparent that an upward motion of the seatalso results in an aft-ward motion, with the relationship between thetwo being dependent on the vertical position of the seat.

In FIG. 21 the inputs and outputs for the various blocks aretwo-dimensional vectors. Thus, the control signal to be provided to thereal plant 16 includes components for controlling two different forceactuators. Before being applied to the real plant 16, the control signalis passed to a decoupling similarity transformation matrix R. Details ofthe procedure can be found in section 3.3 “vector spaces,” of “ControlSystem Handbook,” published by IEEE press.

For the case shown in FIG. 20, in which the coupling is purelykinematic, a similarity transformation is used to decouple thekinematically-cross-coupled plant. In this case, the elements of thedecoupling matrix are real-valued constants. The values of thoseelements can be derived directly from considering the geometry of thesupport.

In other cases, the coupling between motion in two directions includes adynamic, as well as a kinematic component. In such cases, the elementsof the decoupling matrix are complex-valued functions of frequency. Ingeneral, such matrices may not yield low-order realizabletransfer-function matrices suitable for controller implementation.

Another approach to suppressing vibration of a plant in two directionsis to use a fully populated controller matrix rather than a diagonalmatrix for the acceleration loop controller. In this case, the elementsof the acceleration loop controller matrix are computed such that theclosed loop matrix transfer function associated with the accelerationloop is either diagonal or has negligible off-diagonal elements.

Other implementations are within the scope of the following claims:

The invention claimed is:
 1. An apparatus comprising: an activesuspension including an electromagnetic actuator coupled to a plant in avehicle; a force bias eliminator having the properties of a lowstiffness adjustable spring coupled to the plant for causing theactuator to experience a zero-mean load; a vibration isolation block forgenerating a control signal based on the response of a nominal plant tomeasured disturbances of the actively suspended plant, and acompensation system for modifying the control signal in response to adifference between the response of the nominal plant to the controlsignal and a measured response of the actively-suspended plant to thecontrol signal.
 2. The apparatus of claim 1, further comprising a sensorfor providing information indicative of a state of the plant.
 3. Theapparatus of claim 1, further comprising a weight sensor for measuring aweight associated with the plant.
 4. The apparatus of claim 1, whereinthe force bias eliminator comprises a pneumatic system.
 5. The apparatusof claim 1, wherein the active suspension is configured to have aplurality of operation modes, including an active mode and a passivemode.
 6. The apparatus of claim 5, wherein the apparatus is configuredto automatically switch between the active mode and the passive mode inresponse to detection of an event.
 7. The apparatus of claim 5, whereinthe apparatus is configured to switch between the active mode and thepassive mode in response to a user selection.
 8. The apparatus of claim1, wherein the plant comprises a seat, and wherein the electromagneticactuator is coupled to the seat for exerting a force on the seat.
 9. Theapparatus of claim 8, wherein the compensation system is configured tomodify the control signal in response to differences in the weight ofdifferent occupants of the seat.
 10. The apparatus of claim 1, whereinthe compensation system comprises a feedback loop carrying a feedbacksignal indicative of plant acceleration; and wherein the compensationsystem is configured to maintain a constant bandwidth of the feedbackloop.
 11. The apparatus of any of claims 1, wherein the compensationsystem is configured to modify the control signal in response to a rapidchange in the weight of the plant.
 12. The apparatus of claim 11,wherein the rapid change in the weight of the plant is caused by aloading or unloading of the plant.
 13. The apparatus of claim 11,wherein the control signal is modified so that the maximum actuatorforce exerted is reduced while the force bias eliminator is allowed toadapt to the new plant weight.
 14. The apparatus of claim 1, wherein thecompensation system is arranged to maintain a constant open loopacceleration transfer function, independent of plant differences. 15.The apparatus of claim 1, wherein the compensation system does notmodify the control signal if the control signal would cause the actuatorto exert forces with a magnitude on the order of frictional forces. 16.The apparatus of claim 1, wherein the compensation system does notmodify the control signal if the control signal would cause theacceleration of the plant to be less than a predetermined threshold.